A known method for recording temperatures is the insertion of temperature dependent resistors such as NTC resistors into, or adjacent to, the item whose temperature is being monitored. A resistor of this type is connected to a constant source of voltage by way of a dropping resistor whereby the voltage drop at the temperature dependent resistor is a gauge for the recorded temperature or, as the case may be, change in temperature.
Temperature dependent resistors change their value in orders of magnitude to the power of ten. Thus, within a temperature range of -60.degree. to +200.degree. C., an NTC resistor changes its value from 160 KOhm to 20 Ohm (8000:1).
In order to achieve a sufficient degree of exactness in measuring at high temperatures, a low ohmic dropping resistor must be used. As can be gathered from curve 1 in FIG. 1, the change in voltage with a low ohmic dropping resistor in the range from -60.degree. C. to 0.degree. C. is almost nill, while in the case of high temperatures, there are sufficient changes in voltage which make a corresponding determination of temperature possible.
If a high ohmic dropping resistor is inserted to record lower temperatures, this results in curve 2, shown in FIG. 1, which is relatively flat in the higher temperature range so that an exact determination of temperatures in this range is not possible.
In practical use, further difficulties may be encountered including disorders such as breaks in conduction or short-circuiting. In the case of a low ohmic dropping resistor, an interruption in conduction to the NTC resistor in the low temperature range cannot be recognized, while in the case of a high ohmic resistor, a short-circuit of the NTC resistor in the high temperature ranges cannot be recognized.